Portable receivers such as selective call receivers, pagers, personal messaging units (PMUs), and the like generally utilize a single cell battery for operation having a voltage in the range of 1.1 to 1.5 volts. In the design of selective call or paging receivers, it is desirable to prolong the operating time between battery charges or replacement. The physical size of paging receivers has reduced over the years. The size and electrical capacity of their batteries have also been reduced, potentially causing a reduction in the paging receiver's operating time. To compensate for the reduced battery capacity, circuitry has been developed to reduce the power drain of the paging products. Special battery saving techniques have been developed such that the only portions of the paging receiver are switched ON for brief intervals determined by the selective call coding protocol.
These devices can contain components which require more operating voltage than is available from a 1.4 V single cell battery. Accordingly, such devices require DC to DC conversion to provide the operating voltage needed for these components, such as a CMOS microprocessor, controller or LCD panel. DC to DC converters are used to boost the battery voltage to a converted voltage high enough to operate these components.
Inductive DC to DC converters have been used in the industry to boost battery voltage. One such converter is illustrated in U.S. Pat. No. 5,028,861 to Pace, assigned to the assignee of the present invention, generally described below. The DC to DC converter regulates the maximum current through an inductor. It operates within a paging receiver and boosts a voltage from a 1.4 V DC single cell battery to substantially 3.1 V DC in order to operate the circuits which require more voltage than provided by the single cell battery. The disadvantages of using inductive DC to DC converters versus capacitive DC to DC converters is that inductors are less efficient, larger, and cost more than capacitors. Inductors are also less reliable, since they are more susceptible to shock and vibration.
Other technologies, such as CMOS, have been investigated to increase the conversion efficiency. Current CMOS capacitive voltage doublers (DC-DC converters) use capacitors and alternate precharge and dump phases to generate double the battery voltage. For battery voltage VBB, the doubler increases this voltage to 2 VBB. A voltage regulator can then be used to lower this voltage to the required level for operating the various components. However, these systems require higher battery voltage during start up under heavy load current operation. Accordingly, a major problem with CMOS circuitry is establishing a voltage high enough to start the operation.
This high voltage requirement has forced the use of bipolar start (switching) circuits since their low turn on voltage can provide direct operation from a single cell battery. An example of a low power start circuit using bipolar technology is described in U.S. Pat. No. 4,961,006 to Pace et al., (the '006 patent), assigned to the assignee of the present application, generally described below.
The inductively loaded switching transistor of the '006 patent used in a DC to DC converter includes an inductive load and a coil switching transistor (NPN transistor) has a first emitter coupled to a first source of a supply voltage and a second emitter coupled to ground via a resistor. The collector of the transistor is coupled via an inductive load to a second source of supply voltage for conducting current flowing therethrough when the switching transistor is on. A drive circuit is provided which is coupled to the switching transistor for supplying a drive current thereto, and feedback is provided for adjusting the amount of base drive supplied to the switching transistor from the drive circuit.
A problem with these bipolar devices is that they require base drive currents to operate. Accordingly, the efficiency of the DC to DC conversion is degraded in high load current operation and is severely degraded in low load current operation.
Thus, there is a need in the art for boosting low battery voltage to a level sufficient to operate a device without requiring base drive currents to operate.